Add examples / basic documentation

[grosser]

I wanted to play around with it, but there are no examples or any runnable code to be found, please add something :)

[xman]

Hi, thanks for your interest to try out. You can find the samples in the development branch, see

https://github.com/xman/sgc-ruby-cuda/tree/devel/samples/rubycuda

[grosser]

Thanks, please add it to master or link them in the readme, so others can find them too.